ON THE TIME TAKEN IN TRANSMISSION OF REFLEX IMPULSES IN THE SPINAL CORD OF THE FROG

By means of photographic records taken with the capillary electrometer, it has been shown that:— 1. The delay in the normal cord of the frog, in the same‐limb reflex, as determined by the interval of time between the two electrical responses of a particular spot of a muscle (the gastrocnemius) when the efferent and afferent fibres of the mixed nerve supplying it (the sciatic) were simultaneously excited by a single break induction shock, varies in different preparations between 0·012 and 0·022 second. It is very rarely as short as 0·012 second, and only occasionally longer than 0·022 second. These numbers refer to cord delay alone, time for transmission along the known length of nerve having been deducted on the assumption that an impulse travels at the rate of 30 metres a second along fresh frog's nerve. 2. When the cord and the cord alone has been acted upon by a very weak dose of strychnine, this delay is somewhat diminished. It then varies in different preparations between 0·009 and 0·020 second at room temperatures, though it is rarely as short as 0·009 second. It is seldom longer than 0·020 second, except when the circulation as well as the cord has been affected by the drug. In such cases the cord delay may become as long as 0·033 second, and the central stimulus may fail to exert its full strength when it first begins to affect the muscle. 3. Alteration in the strength of the artificial stimulus applied to the nerve does not alter the delay in either the normal or the strychnised cord. This statement is certainly true for stimuli above a certain strength (varying with the sensitiveness of the preparation), but probably does not apply to stimuli the strength of which is only just above the threshold value. The difficulties in the way of determining the threshold value for any one preparation have so far prevented direct satisfactory investigation of the effects of just adequate stimuli. 4. Cooling the cord by applying ice to the back of the preparation greatly increases the delay in the strychnised cord. Cold seems also to increase to some extent the delay in the normal cord. 5. Repeated stimulation (fatigue) may lengthen the delay in the normal cord. 6. The direct action of strychnine on the cord consists principally, so far as the same‐limb reflex is concerned, in making the discharge stronger, and eventually longer, in response to a weaker stimulus. So far as the crossed reflex is concerned, it also brings about something which before prevented an effectual response from being obtained in the muscle to a stimulus so brief as a single induction shock, applied to the sciatic nerve of the opposite limb. 7. In strychnine preparations, from which an effectual response can only just be obtained when the nerve of the opposite limb is stimulated, the cord delay is always roughly about double what it is when the nerve of the same limb is stimulated. 8. Whereas the delay in the case of the same‐limb reflex is only somewhat shortened by the action of strychnine on the cord, the extra delay in the case of the crossed reflex may be very considerably shortened by the continued action of the drug, so long as its action is confined to the cord. It may be reduced to half, or even to a fifth of what it was, in the course of an experiment in which the drug was taking effect rapidly. It seems, however, never to be reduced to less than 0·004 second, and seldom becomes so short as this. It may vary in any one preparation independently of the whole delay in the simpler reflex, but if the circulation has been affected by the drug it also is long. 9. The extra delay in the crossed reflex is no more affected by the strength of the stimulus applied to the nerve than is the whole delay in the same‐limb reflex. It is affected, and in the same way, by changes in the temperature of the cord. 10. The cord delays in the uncrossed and in the crossed reflex are of the same order when the excitability of the cord has been raised by phenol instead of by strychnine, and bear the same sort of relation to one another, provided that in the case of the crossed reflex the strength of the artificial stimulus is not very greatly in excess of (not more than five times) the strength just necessary to produce it. It has been inferred:— 11. That in the same‐limb reflex there is normally a single synapse interposed in the conductive path of each individual fibre concerned, and that the time taken to pass it in the normal animal probably lies between 0·010 and 0·020 second. 12. That in the crossed reflex investigated there are normally two synapses interposed in the conductive path of each individual fibre concerned, and that the time taken to pass the additional (“secondary”) one is about the same as that taken to pass the primary one in a normal animal, but that it is subject to greater variation in each individual cord, and that there is, determining it, something which is more susceptible to such agents as drugs and fatigue. An abstract of this communication, under a somewhat different title, appeared in the Proceedings of the Royal Society, B., vol. lxxix., 1907, p. 503.